Basic Concepts of NAT

There are several situations where we need address translation such as, a network which do not have sufficient public IP addresses want to connect with the Internet, two networks which have same IP addresses want to merge or due to security reason a network want to hide its internal IP structure from the external world. NAT (Network Address Translation) is the process which translates IP address. NAT can be performed at firewall, server and router. In this assignment we will understand how it is performed at Cisco router.

NAT Terminology

Before we understand NAT in details let’s get familiar with four basic terms used in NAT.

Term	Description
Inside Local IP Address	Before translation source IP address located inside the local network.
Inside Global IP Address	After translation source IP address located outside the local network.
Outside Global IP Address	Before translation destination IP address located outside the remote network.
Outside Local IP Address	After translation destination IP address located inside the remote network.

Types of NAT

There are three types of NAT; Static NAT, Dynamic NAT and PAT. These types define how inside local IP address will be mapped with inside global IP address.

Static NAT

In this type we manually map each inside local IP address with inside global IP address. Since this type uses one to one mapping we need exactly same number of IP address on both sides.

Dynamic NAT

In this type we create a pool of inside global IP addresses and let the NAT device to map inside local IP address with the available outside global IP address from the pool automatically.

PAT

In this type a single inside global IP address is mapped with multiple inside local IP addresses using the source port address. This is also known as PAT (Port Address Translation) or NAT over load.

Situations where NAT is used

There are no hard and fast rules about where we should use NAT or where we should not use the NAT. Whether we should use the NAT or not is purely depends on network requirement for example NAT is the best solution in following situations: -

·       Our network is built with private IP addresses and we want to connect it with internet. As we know to connect with internet we require public IP address. In this situation we can use NAT device which will map private IP address with public IP address.

·       Two networks which are using same IP address scheme want to merge. In this situation NAT device is used to avoid IP overlapping issue.

·       We want to connect multiple computers with internet through the single public IP address. In this situation NAT is used to map the multiple IP addresses with single IP address through the port number.

Advantages and disadvantages of NAT

Nat provides following advantages: -

•   NAT solves IP overlapping issue.
•   NAT hides internal IP structure from external world.
•   NAT allows us to connect with any network without changing IP       address.
•    NAT allows us to connect multiple computers with internet through the single the public IP address.

NAT has following disadvantages: -

•   NAT adds additional delay in network.
•   Several applications are not compatible with NAT.
•   End to end IP traceability will not work with NAT.
•   NAT hides actual end device.